The disclosed embodiments relate to frequency measurements and the detection of digital data using digital signal processors (DSP) in frequency shift keying (FSK) based communication systems. Specifically, the invention relates to a communications system and method for transmitting and receiving digital data using FSK and frequency modulation (FM) techniques on narrowband or very narrowband channels.
Most basic communication systems use frequency references. A frequency reference is a stable signal (source) with a constant frequency. It can be a ceramic resonator or a quartz crystal oscillator. The ceramic resonator is often used instead of crystal because of its lower price. The frequency accuracy is not as good as a crystal but in many applications the accuracy is not critical. The ceramic resonator is a two terminal device with impedance containing both a real and reactive elements.
Quartz crystals are also used in communication systems and are based on the piezoelectric effect, and they can give an accurate frequency to be used for example as a LO (local oscillator) signal in a super heterodyne receiver. The electronics inside a personal computer are often synchronized to a quartz crystal clock signal.
The quality of the reference frequency required depends on the application. High precision is often required in measurement applications and the reference frequency should be both extremely accurate and very precise. There are a few common ways to provide a good reference. For example, using a Global Positioning System (GPS) satellite signal or the TV the line frequency signal can be used. If temperature stability is a requirement, quartz crystal may be put inside a thermally insulated enclosure which is called an oven. Another way to enhance temperature stability is to use a Voltage Controlled Temperature Compensated Crystal Oscillator (VCTX), which is an oscillator based on a tuning circuit which keeps the frequency stable. VCTXOs can be quite small integrated parts and often used in GSM mobile equipment i.e. mobile phones.
Frequency counters use a digital counting technique to determine the frequency of an unknown signal. A frequency counter consists of an accurate clock signal source, a gate circuit and a digital counter. Frequency counters can be divided in three main categories: heterodyne counters, transfer oscillator (or phase lock) counters and direct digital counters.
The direct digital counter counts pulses as long as the gate circuit is open. The gate is kept open, for example exactly for one second, by clocking it with the accurate clock or gate signal. As mentioned, the counter counts pulses into a register until the gate is closed. The unknown frequency can then be calculated simply by dividing the number of the pulses counted by the time the gate circuit was open. This kind of frequency counter is called a direct digital counter.
The input frequency range of a frequency counter can be extended by adding a prescaler between the signal input and the counter. The prescaler is a digital (binary counter) circuit that divides the frequency of the measured signal by some constant, e.g. 1000.
Frequency counters built with fast digital integrated circuits are able to measure frequencies of up to 1 GHz. The limit is set by the maximum speed of the digital logic. Prescalers used in commercial frequency counters are able to extend the frequency range to about 3 GHz. Using GaAs (Gallium Arsenide) circuits it's possible to build prescalers that are able to manage 10 GHz. However, this frequency measurement method is not suitable for high data rate FM communication system as it requires transmitting many cycles per bit.
Phase locked frequency counters are based on phase locking the measured signal to a low-frequency voltage controlled oscillator (VCO). The low-frequency signal can be measured with a normal counter (for example with the direct digital counter) and the frequency of the measured signal is then N times the frequency shown on the counter output, where N is the order of the harmonic frequency to which the oscillator was locked to. The phase lock counter uses a comb generator to generate harmonic frequencies. A harmonic frequency is then filtered out from the comb spectrum and fed into a mixer. If the mixture is not equal to zero, the frequency is tuned by changing the control voltage of the VCO. Upon locking, the measured frequency is N times the frequency measured with the counter.
The phase locked frequency counter has two drawbacks. First, its resolution is reduced by factor N compared to a direct digital counter measuring the same range. Second, continuous phase locking of a frequency modulated (FM) signal may be difficult, at least when the frequency deviation of the input signal is wide.
A heterodyne frequency counter uses heterodyne mixing to extend the frequency range of a direct digital counter. In a heterodyne counter the measured signal is transferred to a lower frequency band by mixing it with an extremely stable local oscillator (LO).
A comb generator creates harmonic frequencies from which one can be selected (with a narrow (switched) filter) and filtered out. The filtered harmonic frequency is then fed into a mixer. The mixture will generate the difference frequency which can easily be measured with a direct digital counter and then unknown input frequency is calculated.
High frequency measurement using counters takes a relative long time for counting and determining the frequency, which has the effect of reducing the data rate. Also this heterodyne technique limits the data rate with the resulting low (Beat) frequency signal, as a relatively long time is needed to measure one cycle of this frequency. Accordingly, this method is not well suited for high data rate FM communication systems.
Digital Phase Locked Loop (DPLL) technology uses a digital phase locked loop to measure the frequency of a high frequency signal, and represents the state of the art in high frequency measurement, measuring the frequency in one or two cycles of the received frequency.
U.S. Pat. No. 6,630,820 describes a method and apparatus for measuring instantaneous frequency of FM modulated signal, however this technique of digitizing the FM signal and computing the instantaneous frequency is not fast enough for high speed digital FSK-NBFM applications.
The automatic tunable band pass filter (BPF) may be designed with an internal PLL to track the band at which the power is concentrated and when the BPF locks on the band, the data can be determined. The advantage of using such a method is the hardware reduction that would result if one tunable BPF can be used to track more than one carrier at the same time, i.e., switching or time division multiplexing (TDM).
Although U.S. Pat. No. 5,757,858 discloses a dual mode digital communication system based upon a frequency modulated (FM) mode and a code division multiple-access (CDMA) mode, it fails to disclose a digital FSK method.
Similarly, although U.S. Pat. No. 6,484,112 describes a method for estimating the frequency of a time signal by means of a discrete Fourier transformation and interpolation, it fails to analyze multi-sample messages at high speed in order to determine the peak of spectral power density.